This invention relates generally to laser scribers which are employed to scan thin work sheets, such as semiconductor wafers, with a laser beam, and more particularly to laser scanning mechanisms of the type having a stationary laser head which are operable to move the work sheet relative to the stationary laser beam.
Laser scribers are conventionally used to form scores or shallowly notched lines in the surface of thin sheets, such as silicon wafers with integrated circuits (IC) previously formed thereon, and ceramic sheets, by irradiating the work surface with a narrowly focused laser beam, thereby to facilitate the splitting or severing of such thin sheet works into pellets of desired size.
Such severing is performed for the purpose of separating a large number of IC or other microcircuit elements formed on a common thin sheet from each other, and is usually effected along two sets of regularly spaced lines extending at right angles to each other. In order to enable such severing operation, the work sheet must, of course, be scanned with a laser beam along each line of separation.
The scanning operation may be performed by displacing the laser head in the same manner as a diamond edged cutter used in conventional scribers but, with laser scribers, such design is generally undesirable due to the limited strength of the laser head structure to withstand mechanical vibration. It is thus recommended for ease and simplicity in design and for efficiency of operation, to displace the work sheet relative to a stationary laser head.
On the other hand, microcircuit elements such as IC formed on a silicon wafer are closely arranged thereon in rows at right angles to each other, as stated above, thereby leaving an allowance of only a few ten microns for scoring. This requires that the scanning of the laser beam be effected along a practically true straight line without any deflection therefrom.
In the use of a laser scriber of the type including a stationary laser head and a work-carrying mechanism in which two tables are movable in respective planes parallel to the plane of the work sheet and in respective X and Y directions at right angles to each other, the common practice has previously been to place one of the two tables at rest simply by rendering drive means provided for the table ineffective or by interrupting the driving connection therewith. In this arrangement, the table that is presumably at rest is unavoidably guided with some lateral play to cause undesired scanning deflection under the effect of machine vibration or other external disturbances. In other words, it has been impossible in the prior art laser scanning arrangements to avoid oscillatory or other deflection of the actual line of scan from its intended course during scanning in either of the two (X and Y) directions. For example, in scoring a silicon wafer with IC formed thereon with a conventional scanning mechanism, the laser irradiation is often extended beyond the limits of scoring allowance to impair the IC region of the wafer, thus providing damage that results in an increased number of rejects in the IC products finally obtained.
In an attempt to deal with this problem, it has been proposed to design work-carrying structure that is capable of displacing the work sheet relative to the laser beam with exceptionally high accuracy. A structure capable of operating in this manner must, however, inevitably be of increased weight and inertia, making it difficult to effect high-speed scanning and to form a factor of scanning deflection due to the increase in driving effort required therefor.